(1) Field of the Invention
This invention relates to an electrical insulating oil and oil-filled electrical appliances impregnated therewith. More particularly, the invention relates to an electrical insulating oil which is prepared from a heavier product that is obtained in the ethyltoluene production process.
(2) Description of the Prior Art
Oil-filled electrical appliances such as oil-filled capacitors recently have a marked tendency to be made small in size and light in weight. In compliance with this trend, various kinds of plastic materials have been developed as insulating materials and dielectric materials. They are used in place of or in combination with the conventionally used insulating paper.
In connection with electrical insulating oils to be used for impregnating the oil-filled electrical appliances, several problems have arisen with the increasing use of the plastic materials. That is, the compatibility of conventional electrical insulating oils such as refined mineral oils, polybutenes and alkylbenzenes with plastic materials such as polyolefins is not always satisfactory. The conventional electrical insulating oils tend to dissolve or swell the plastic materials and sometimes impair the dielectric strength of oil-filled electrical appliances.
Accordingly, proposals on improved electrical insulating oils for impregnating oil-filled electrical appliances using plastic materials such as polyolefin, are eagerly awaited.
For the purpose to produce ethylbenzene, ethyltoluene and cumene, it has been widely put into industrial practice that hydrocarbons such as benzene and toluene are alkylated with olefins such as ethylene and propylene in the presence of alkylation catalysts to obtain alkylbenzenes. In this process, various kinds of alkylation catalysts such as aluminum chloride, solid phosphoric acid, and boron fluoride are used.
In the above alkylation process, a heavier by-product fraction containing diarylalkanes, triaryldialkanes and other impurities having chemical structures that correspond to the kinds of starting materials and alkylation catalyst, is obtained. For example, as disclosed in U.S. Pat. Nos. 4,108,788; 4,111,824 and 4,111,825, it is known that the heavier product that is obtained from a process to alkylate benzene with ethylene in the presence of aluminum chloride catalyst, can be used as an electrical insulating oil for general purposes.
The electrical insulating oil comprising the above fraction disclosed in the foregoing references is, however, not always satisfactory as an impregnating oil for oil-filled electrical appliances because the pour point of the fraction is relatively high and it contains tarry substances, unsaturated compounds and carbonyl compounds that cannot be easily removed only by distillation. Even when it is treated by refining, it is not suitable for use under severe conditions. Furthermore, because it swells plastic materials such as polyolefin to a considerable extent, it is not suitable for impregnating oil-filled electrical appliances that are made by using plastic materials.
As described above, the electrical insulating oils of this kind are not satisfactory because they contain tarry substances, unsaturated compounds and carbonyl compounds that cannot easily be removed by distillation. Furthermore, the compatibility of them with plastic materials is not good.